Bipolar collectors for a fuel cell of the PEM type

ABSTRACT

A bipolar collector-heat exchanger is shown which combines, at the same time, a main structure made of an electronically and thermally conducting material, discrete collection of the charges and the heat, carried out by metal contacts which are distributed uniformly at the surface of the electrodes and penetrate into the main structure, but which do not pass fully through it, and the use of non-conductive open structures which homogenise the flow of the gaseous reactants.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation application, which is being filed as the national phase of International Application No. PCT/FR00/02660 filed Sep. 27, 2000, which claims priority of French Patent Application No. 99.12339 filed Sep. 30, 1999 and is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to a bipolar collector-heat exchanger, characterised in that it combines, at the same time, a main structure made of an electronically and thermally conducting material, discrete collection of the charges and the heat, carried out by metal contacts which are distributed uniformly at the surface of the electrodes and penetrate into the main structure, but which do not pass fully through it, and non-conductive open structures which permit homogenisation of the flow of the gaseous reactants.

[0004] 2. Description of the Related Art

[0005] Research relating to the development of bipolar collectors for cells, in particular of the PEM-FC type, having advantageous technical and economic characteristics, has been highly active for a number of years. Initially, efforts were directed at the selection of collectors with a much lower cost than the collectors which are produced by machining a graphite plate. From this point of view, the most advantageous results were obtained by developing polymer-carbon plates, the main advantage of which resides in the possibility of processing them by moulding, leading to very low manufacturing costs.

[0006] On the other hand, other solutions consisting in the use of ribbed metal strips or metal foams have proved to be more demanding, in so far as the surface of these metallic materials necessarily has to be coated with a compound which prevents both corrosion and passivation.

[0007] Concerning the ohmic drop resulting from the passage of the current through the collectors, it should be noted that the resistivity of the polymer-carbon composites is between 3×10⁻² and 1 Ω.cm, whereas graphite has a resistivity of the order of 1.3×10⁻³ Ω.cm. The result of this, for bipolar collectors such as those schematically represented in FIG. 1, is that the ohmic drop caused by an apparent current density of 0.5 A.cm⁻² is less than 1 mV when the collector is made of graphite, and is 20 mV for a composite collector having a resistivity of 0.05 Ω.cm.

[0008] In fact, for many uses in which the current density does not actually exceed 0.5 A.cm⁻² it may be considered that a loss of 20 mV, on a voltage of the order of 0.7 V, remains acceptable but without being negligible. On the other hand, the situation is entirely different when it is considered that the expected progress in the field of PEM-FCs will lead to operation with current densities at least equal to 1 A.cm⁻².

[0009] Lastly, this problem needs be fully reconsidered when the bipolar collectors are also intended to constitute heat exchangers. Under these circumstances, it can be shown that the dimensions of these collector-exchangers, associated with the constraints linked with good diffusion of heat in the materials constituting them, mean that the choices in terms of the nature and structure of the collector-exchangers have to be reconsidered.

[0010] For instance, in the case of a collector-exchanger whose diagram is represented in FIG. 2, made of a composite material with a resistivity of 0.05 Ω.cm, the ohmic drop amounts to 50 mV for a current density of 0.5 A.cm⁻², i.e. a loss of 7% on the voltage of an element operating at 0.7 V.

[0011] However, for applications such as the powering of electrical vehicles, the rated power required being for example 50 kW with a total voltage of 140 V, it can be seen that the power of an element is of the order of 250 W, i.e. an energy of close to 200 W to be discharged per element. Under these conditions (and even for cells having a power which is twice as low), it is necessary for all the bipolar collectors to be heat exchangers as well.

[0012] For power cells, the collector-exchanger ought therefore to be regarded not as a particular case, but as the standard component.

[0013] We have already shown (Patent Application FR 99 04277) the benefit provided by discrete collection of the charges by metal needles which pass partly through the bipolar collectors. The device made it possible both to lighten the collectors and to minimise the ohmic drops in these collectors. This concept has been re-adopted and appropriately adapted for the production of collector-exchangers. We have observed that, even though the cross section of the needles is small compared with the apparent area of the electrodes, these needles could constitute efficient means for draining the heat generated at the electrodes.

SUMMARY OF THE INVENTION

[0014] The present invention relates to a bipolar collector-heat exchanger for a fuel cell, characterised in that it combines, at the same time:

[0015] a main structure made of an electronically and thermally conducting material,

[0016] discrete collection of the charges and the heat, carried out by metal contacts which are distributed uniformly at the surface of the electrodes and penetrate into the main structure, but which do not pass fully through it,

[0017] the use of non-conductive open structures which homogenise the flow of the gaseous reactants.

[0018] According to a first embodiment, the bipolar collector-heat exchanger according to the invention is characterised in that the electronically and thermally conductive main structure consists of two parts, at least one of which comprises, on one of its faces, ribs which permit the circulation of a heat-exchange fluid.

[0019] In the event that both parts of the main structure comprise ribs on one of their faces, the said faces are juxtaposed during their assembly in order to create channels which permit circulation of the heat-exchange fluid.

[0020] According to another characteristic of the invention, the metal contacts are metal needles which do not pass fully through each part of the main structure, but instead penetrate into the protruding parts contained between the channels, their end being located between 0.4 and 0.9 mm from the plane separating the two parts constituting the main structure. These needles penetrate into the electrodes by a depth of between 0.1 and 0.2 mm, and they are distributed uniformly at the surface, in square or rectangular patterns.

[0021] The bipolar collector-heat exchanger according to the invention includes, between each electrode and the surface of the collector-exchanger, a space which is defined by studs placed on the outer faces of the two parts which constitute the main structure. This space, whose thickness is between 0.8 and 2 mm, is filled with a non-conductive open structure.

[0022] This non-conductive open structure consists:

[0023] either of an insulating polymer material whose surface preferably has a hydrophobic nature,

[0024] or of a foam whose core consists of a heat-conducting metallic material, coated with an anticorrosion layer having a hydrophobic nature,

[0025] or of a longitudinally open polymer mesh whose surface preferably has a hydrophobic nature.

[0026] According to another embodiment, the collector-exchanger according to the invention is characterised by the use of an electronically and thermally conductive main structure consisting of two parts, at least one of which comprises, further to the studs which are made on one of their faces and are used to define the space between the electrodes and the collector-exchanger, on their other face, cylindrical studs with a height of between 0.5 and 2 mm, having a diameter of between 3 and 10 mm. The two parts, when juxtaposed so that the studs face one another, create a space which is intended for circulation of the heat-exchange fluid.

[0027] According to one of the characteristics of the invention, the cylindrical studs which are used to make a space intended for circulation of the heat-exchange fluid are distributed uniformly at the surface of the collector-exchanger, in a number such that the total area of their cross section occupies at least 40% of the apparent (visible) area of the electrodes.

[0028] According to another characteristic of the invention, the cylindrical studs which are used to make a space intended for circulation of the heat-exchange fluid are distributed uniformly at the surface of the collector-exchanger, in a number such that the total area of their cross section is less than 10% of the apparent area of the electrodes. In this case, the space intended for circulation of the heat-exchange fluid is filled with an open-structured foam consisting of a metallic material which is a good conductor of heat and electricity, the strands of which are in contact with the walls of the main structure. The constituent material of the foam is copper or nickel.

[0029] According to another characteristic of the invention, the walls in contact with the electronically and thermally conductive foam are coated with a thin electronically conductive layer such as a metal, a metal alloy or a conductive varnish.

[0030] According to the second embodiment which is described, the metal contacts which carry out the discrete collection of the charges and the heat are needles which penetrate partially into the main structure, their end being located between 0.4 and 0.9 mm from the plane separating the two parts constituting the main structure. These needles penetrate into the electrodes by a depth of between 0.1 and 0.2 mm, and they are distributed uniformly in square, rectangular or equilateral-triangular patterns.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Other characteristics and advantages of the present invention will become more clearly apparent on reading the following description, with reference to the appended FIGS. 3 to 6 in which:

[0032]FIG. 3 represents a sectional view of the central part of a bipolar collector-heat exchanger according to a first embodiment,

[0033]FIG. 4 represents a section on the axis M-M′ of the bipolar collector-heat exchanger represented in FIG. 3,

[0034]FIG. 5 represents a sectional view of the central part of a bipolar collector-heat exchanger according to a second embodiment,

[0035]FIG. 6 represents a plan view of the bipolar collector-heat exchanger represented in FIG. 5.

[0036] The various figures are represented without taking account of the scales of the drawings.

DETAILED DESCRIPTION OF THE BEST MODES OF THE INVENTION

[0037] The present invention results from a compromise between

[0038] minimising the ohmic drops,

[0039] lightening the collectors,

[0040] good heat exchange,

[0041] simplified manufacture.

[0042] Two embodiments have been studied in particular, each giving rise to several variants.

[0043] First Embodiment

[0044]FIG. 3 represents simply the central part of the collector-exchanger to which the present invention relates, excluding the borders where the fluid inlets and the sealing gaskets are located.

[0045] The bipolar collector-heat exchanger essentially consists of two symmetrical parts E and E′, the juxtaposition of which on the plane M-M′ forms the channels F. The constituent material of these two main parts is a polymer-carbon composite whose resistivity is 0.1 Ω.cm. It is, for example, the material marketed under the reference EMI 0683 by the company RTP France.

[0046] Furthermore, each of these main parts E and E′ includes studs C_(i) and C_(i)′, which bear on the electrodes A and A′ to leave a clear vein or seam D or D′.

[0047] The veins D and D′, with a thickness of between 0.8 and 2 mm, are advantageously filled, as is claimed in the Patent Application FR 98 09236, with an open structure which fulfils the role of a distributor and homogeniser for the gas flows. In the case of a standard collector, the constituent material of the open structure is, for example, a polymer whose surface is advantageously hydrophobic. In order to facilitate the heat transfer towards the central part of the collector-exchangers, it is possible but not obligatory to use, for filling the veins D and D′, a metal foam with a low surface mass density (20 to 30 mg/cm²) covered, on all its strands, with a hydrophobic film which moreover prevents any corrosion of the material constituting the foam, the latter possibly being copper, for example.

[0048] The height of the channels F is between 1 and 4 mm. These channels permit the circulation of a heat-exchange fluid. In order to amplify the heat exchange between the heat-exchange fluid and the collector-exchanger, a copper foam with a low surface mass density (20 to 50 mg/cm²) may be arranged in the channels. Owing to slight compression during assembly, the strands of the foam are in contact with the walls of the collector.

[0049] The thickness I of the solid part of each half-collector is between 0.5 and 1.5 mm.

[0050] The cylindrical studs C_(i) and C_(l)′ have a diameter of between 3 and 4 mm, their height being between 0.8 and 2 mm, and their distribution is such that the total area of their cross section represents only at most 3% of the apparent area of the electrode on which they bear (i.e. two to three studs per 10 cm² of electrode), which limits the masking of the active surface.

[0051] The needles B_(i or B) _(i)′ do not pass through the parts E and E′, but stop at a distance of between 0.4 and 0.9 mm from the plane M-M′. Their diameter is between 0.2 and 0.3 mm. They consist, for example, of stainless steel 316 L. The end which penetrates into the electrode, over a length of between 0.1 and 0.2 mm, is coated with a deposit which prevents passivation and corrosion.

[0052]FIG. 4 shows the distribution of the needles B_(l), and B_(i)′: this is a distribution in rectangular or square patterns, the distance G between B₁ and B₂ generally being greater than the distance H between B₁ and B_(α1) or B₂ and B_(α2).

[0053] The width of the channels F is between 1 and 3 mm. The spacing between two channels is between 1.5 and 2.5 mm (1.5 mm when the width of the channel is 3 mm, and 2.5 mm when it is only 1 mm). For a channel width of 2 mm, the spacing is 2 mm.

[0054] Since the position of the needles is in the middle of the spacing between two channels, the result is that the distance G between two needles (distance B₁-B₂) is between 3 and 4.5 mm. The distance H along the other axis (distance between B₁ and B_(á1)) is determined as a function of the current density at the electrodes; it is between 2 and 4.5 mm.

[0055] The composite-material parts are advantageously obtained by moulding. As for the composite material, it is beneficial to use the product with the reference EMI 0683 (from RTP France), which includes nickel-coated carbon fibres. In order to improve the bond at the interface between the two main parts, this may be surface-treated or a conductive varnish or a metallic film may be deposited there.

[0056] The needles may be inserted into the composite parts by various means,

[0057] insertion into pre-drilled holes,

[0058] hot nailing,

[0059] insertion during moulding.

[0060] In the case of insertion into pre-drilled holes, the electrical contact between the needle and the composite may be improved by interposing a powder or a conductive varnish.

[0061] Second Embodiment

[0062]FIG. 5 corresponds to the second typical embodiment of the invention, and it represents the central part of the bipolar collector-heat exchanger, excluding the borders where the fluid inlets and the sealing gaskets are located.

[0063] There, the collector-exchanger again consists of two main parts E and E′, made of the same electronic-conductor composite material as the one in the first embodiment, which are juxtaposed on a plane separating the studs J_(l) and J_(i)′.

[0064] These studs have a diameter of between 3 and 10 mm, and the number of them is such that the area of their cross section is at most equal to 8% of the apparent area of the electrodes, their distribution being as uniform as possible. The height of the studs J_(i) and J_(i)′ is between 0.5 and 2 mm. A vein or seam, the area of which represents more than 90% of the area of the electrodes and whose height K is between 1 and 4 mm, is located between these studs.

[0065] As in the first embodiment which was described (see FIG. 3), the spaces D and D′ where the gaseous reactants circulate are limited in their height by the studs L_(i) and L_(i)′, the characteristics of which are identical to those given for the previous embodiment.

[0066] In the same way, the spaces D and D′ are filled with an open structure which homogenises the gas flows.

[0067] The essential difference between the two typical embodiments resides, for the second case, in the fact that the internal space permitting the heat exchanges occupies a much larger volume. For this reason, and in order to ensure not only good circulation of the heat-exchange fluid, i.e. good heat exchange, but also good electrical conductivity between the parts E and E′, the central space is filled with a metal foam whose strands are in good contact (by being compressed) with the surfaces of the parts E and E′, which surfaces may advantageously be coated with a metallic film such as copper or nickel.

[0068] As in the previous example, the charges at the electrodes are collected by stainless-steel needles B_(i) whose tips, which penetrate by 0.1 to 0.2 mm into the electrodes A and A′, are coated with a deposit which prevents passivation and corrosion (see the Patent Application FR 98 09236).

[0069]FIG. 6 represents a uniform distribution of the needles B_(i), corresponding to a pattern of the equilateral-triangle type, the distance between two needles, or the side length of the triangle, having a dimension of between 2.5 and 5 mm. The studs L_(i) and L_(i)′ which bear on the electrodes have a diameter such that they are placed between the needles.

[0070] In this configuration, the distance between the needles can therefore be smaller than in the previous example.

[0071] The distance O separating the tip of the needles from the internal space is between 0.4 and 0.9 mm.

[0072] As regards the characteristics of the materials which are used as implementation means (insertion of the needles, for example), the solutions are the same as those described for the first embodiment.

[0073] The invention is not limited to the two embodiments described above, but encompasses all variants thereof. 

1. Bipolar collector-heat exchanger, which combines, at the same time: a main structure made of an electronically and thermally conducting material, discrete collection of the charges and the heat, carried out by metal contacts which are distributed uniformly at the surface of the electrodes and penetrate into the main structure, but which do not pass fully through it, and the use of non-conductive open structures which homogenise the flow of the gaseous reactants.
 2. Bipolar collector-heat exchanger according to claim 1, characterised in that the electronically and thermally conductive main structure consists of two parts, at least one of which comprises, on one of its faces, ribs which permit the circulation of a heat-exchange fluid.
 3. Bipolar collector-heat exchanger according to claim 2, characterised in that, in the event that both parts of the main structure comprise ribs on one of their faces, the said faces are juxtaposed during their assembly in order to create channels which permit circulation of the heat-exchanged fluid.
 4. Bipolar collector-heat exchanger according to claim 1, characterised in that the metal contacts are metal needles which penetrate into the protruding parts contained between the channels intended for circulation of the heat-exchange fluid, their end being located between 0.4 and 0.9 mm from the plane separating the two parts constituting the main structure.
 5. Bipolar collector-heat exchanger according to claim 4, characterised in that the needles penetrate into the electrodes by a depth of between 0.1 and 0.2 mm, and they are distributed uniformly in square or rectangular patterns.
 6. Bipolar collector-heat exchanger according to claim 1, characterised in that a space, whose thickness is between 0.8 and 2 mm and which is filled with a non-conductive open structure, is made between each electrode and the surface of the collector-exchanger, with the aid of studs placed on the outer faces of the two parts with constitute the main structure.
 7. Bipolar collector-heat exchanger according to claim 6, characterised in that the non-conductive open structure is a polymer foam whose surface preferably has a hydrophobic nature.
 8. Bipolar collector-heat exchanger according to claim 6, characterised in that the non-conductive open structure is a foam whose core consists of a heat-conducting metallic material and which is coated with an anticorrosion layer having a hydrophobic nature.
 9. Bipolar collector-heat exchanger according to claim 6, characterized in that the non-conductive open structure consists of a polyer mesh which is open longitudinally and whose surface preferably has a hydrophobic nature.
 10. Bipolar collector-heat exchanger according to claim 1, charactersed in that the electronically and thermally conductive main structure consists of two parts, at least one of which comprises, further to the studs which are made on one of their faces and are used to define the space between the electrodes and the collector-exchanger, on their other face, cylindrical studs with a height of between 0.5 and 2 mm, having a diameter of between 3 and 10 mm. The two juxtaposed parts create a space intended for circulation of the heat-exchange fluid.
 11. Bipolar collector-heat exchanger according to claim 10, characterised in that the cylindrical studs which are used to make a space intended for circulation of the heat-exchange fluid are distributed uniformly at the surface of the collector-exchanger, in a number such that the total area of their cross section occupies at least 40% of the apparent area of the electrodes.
 12. Bipolar collector-heat exchanger according to claim 10, characterised in that the cylindrical studs which are used to make a space intended for circulation of the heat-exchange fluid are distributed uniformly at the surface of the collector-exchanger, in a number such that the total area of their cross section is less than 10% of the apparent area of the electrodes.
 13. Bipolar collector-heat exchanger according to claim 12, characterised in that the space intended for circtulation of the heat-exchange fluid is filled with an open-structured foam consisting of a metallic material which is a good conductor of heat and electricity, the strands of which are in contact with the walls of the main structure.
 14. Bipolar collector-heat exchanger according to claim 13, characterised in that the constitutent material of the foam is made of copper or nickel.
 15. Bipolar collector-heat exchanger according to claims 10 and 13, characterised in that the walls in contact with the foam are coated with a thin electronically conductive layer such as a metal, a metal alloy or conductive varnish.
 16. Bipolar collector-heat exchanger according to claims 1 and 10, characterised in that the metal contacts which carry out the discrete collection of the charges and the heat are metal needles which penetrate partially into the main structure, their and being located between 0.4 and 0.9 mm from the plane separating the two parts constituting the main structure.
 17. Bipolar collector-heat exchanger according to claim 12, characterised in that the needles penetrate into the electrodes by a depth of between 0.1 and 0.2 mm, and they are distributed uniformly in square, rectangular or equilateral-triangular patterns.
 18. Bipolar collector-heat exchanger, which combines, at the same time: a main structure made of an electronically and thermally conducting material, discrete collection of the charges and the heat, carried out by metal contacts which are distributed uniformly at the surface of the electrodes and penetrate into the main structure, but which do not pass fully through it, and the use of non-conductive open structures which homogenise the flow of the gaseous reactants.
 19. Bipolar collector-heat exchanger according to claim 18, wherein the electronically and thermally conductive main structure consists of two parts, at least one of which comprises, on one of its faces, ribs which permit the circulation of a heat-exchange fluid.
 20. Bipolar collector-heat exchanger according to claim 19, wherein in the event that both parts of the main structure comprise ribs on one of their faces, the said faces are juxtaposed during their assembly in order to create channels which permit circulation of the heat-exchange fluid.
 21. Bipolar collector-heat exchanger according to claim 18, wherein the metal contacts are metal needles which penetrate into the protruding parts contained between the channels intended for circulation of the heat-exchange fluid, their end being located between 0.4 and 0.9 mm from the plane separating the two parts constituting the main structure.
 22. Bipolar collector-heat exchanger according to claim 21, wherein the needles penetrate into the electrodes by a depth of between 0.1 and 0.2 mm, and they are distributed uniformly in square or rectangular patterns.
 23. Bipolar collector-heat exchanger according to claim 18, wherein a space, whose thickness is between 0.8 and 2 mm and which is filled with a non-conductive open structure, is made between each electrode and the surface of the collector-exchanger, with the aid of studs placed on the outer faces of the two parts which constitute the main structure.
 24. Bipolar collector-heat exchanger according to claim 23, wherein the non-conductive open structure is a polymer foam whose surface preferably has a hydrophobic nature.
 25. Bipolar collector-heat exchanger according to claim 23, wherein the non-conductive open structure is a foam whose core consists of a heat-conducting metallic material and which is coated with an anticorrosion layer having a hydrophobic nature.
 26. Bipolar collector-heat exchanger according to claim 23, wherein the non-conductive open structure consists of a polymer mesh which is open longitudinally and whose surface preferably has a hydrophobic nature.
 27. Bipolar collector-heat exchanger according to claim 18, wherein the electronically and thermally conductive main structure consists of two parts, at least one of which comprises, further to the studs which are made on one of their faces and are used to define the space between the electrodes and the collector-exchanger, on their other face, cylindrical studs with a height of between 0.5 and 2 mm, having a diameter of between 3 and 10 mm. The two juxtaposed parts create a space intended for circulation of the heat-exchange fluid.
 28. Bipolar collector-heat exchanger according to claim 27, wherein the cylindrical studs which are used to make a space intended for circulation of the heat-exchange fluid are distributed uniformly at the surface of the collector-exchanger, in a number such that the total area of their cross section occupies at least 40% of the apparent area of the electrodes.
 29. Bipolar collector-heat exchanger according to claim 27, wherein the cylindrical studs which are used to make a space intended for circulation of the heat-exchange fluid are distributed uniformly at the surface of the collector-exchanger, in a number such that the total area of their cross section is less than 10% of the apparent area of the electrodes.
 30. Bipolar collector-heat exchanger according to claim 29, wherein the space intended for circulation of the heat-exchange fluid is filled with an open-structured foam consisting of a metallic material which is a good conductor of heat and electricity, the strands of which are in contact with the walls of the main structure.
 31. Bipolar collector-heat exchanger according to claim 30, wherein the constituent material of the foam is made of copper or nickel.
 32. Bipolar collector-heat exchanger according to claim 27, wherein the walls in contact with the foam are coated with a thin electronically conductive layer such as a metal, a metal alloy or a conductive varnish.
 33. Bipolar collector-heat exchanger according to claim 30, wherein the walls in contact with the foam are coated with a thin electronically conductive layer such as a metal, a metal alloy or a conductive varnish.
 34. Bipolar collector-heat exchanger according to claim 18, wherein the metal contacts which carry out the discrete collection of the charges and the heat are metal needles which penetrate partially into the main structure, their end being located between 0.4 and 0.9 mm from the plane separating the two parts constituting the main structure.
 35. Bipolar collector-heat exchanger according to claim 27, wherein the metal contacts which carry out the discrete collection of the charges and the heat are metal needles which penetrate partially into the main structure, their end being located between 0.4 and 0.9 mm from the plane separating the two parts constituting the main structure.
 36. Bipolar collector-heat exchanger according to claim 29, wherein the needles penetrate into the electrodes by a depth of between 0.1 and 0.2 mm, and they are distributed uniformly in square, rectangular or equilateral-triangular patterns. 